1. Field of the Invention
This invention relates to a four stroke engine and more particularly to an air induction system of a four stroke engine, which is most suitable to an outboard motor.
2. Description of Related Art
Recently, some outboard motors incline to utilize four stroke engines. One reason for this tendency is that emissions from the four stroke engines are clean rather than those of two stroke crankcase compression engines. Also, usually the engines have multiple cylinders in order to produce relatively large power. The respective cylinders are spaced vertically relative to each other in those engines for outboard motors. That is, cylinder bores extend generally horizontally.
An air induction system is provided for introducing air charge to combustion chambers in the cylinders. The air induction system has air intake ducts extending generally horizontally along the cylinder bores and a common plenum chamber placed upstream of the air intake ducts. It is desirable to make the air intake ducts proper lengths for improving engine power, particularly the torque characteristic under acceleration conditions from low or medium speeds by using the inertia charge effect. Also, the plenum chamber has a certain volume and a height so that the air intake ducts are connected thereto.
On the other hand, however, the engine is encircled with a protective cowling and a number of engine components must be placed in narrow room formed between the engine body and the protective cowling. Under the circumstances, it is a problem how to make sufficient space for placing the engine components as well as the air intake ducts and the plenum chamber.
It is, therefore, a principal object of this invention to provide a four stroke engine wherein a certain space can be available for placing engine components other than the air intake ducts and the plenum chamber.
Also, as described above, the air intake ducts must have certain lengths. In the meantime, usually a throttle valve for admitting air charge to combustion chambers is contained in a throttle body placed upstream of the plenum chamber. Due to this arrangement, lengths between the throttle valve and the respective combustion chambers tend to be relatively long. Thus, the engine cannot response so quickly to the operator""s desire. Accordingly, the operator is likely to have bad feeling in engine operation.
It is, therefore, another object of this invention to provide a four stroke engine that can response quickly to the operator""s desire in engine operation.
In accordance with one aspect of this invention, a four stroke internal combustion engine comprises a plurality of generally horizontally extending bores having their axis spaced vertically relative to each other. A plurality of pistons are provided and each piston reciprocates within a respective one of the cylinder bores. A cylinder head closes one end of the cylinder bores. Each of the cylinder bores, the pistons and the cylinder head generally defines a respective combustion chamber for burning an intake charge. The cylinder head has a plurality of air intake passages communicating with the combustion chambers for supplying at least air charge thereto. The engine further comprises a plurality of air intake ducts each connected to a respective one of the air intake passages. Each of the air intake ducts has a generally straight section extending generally horizontally and parallel to each other. The distance between the straight sections is less than the distance between the axes of the cylinder bores.
In accordance with another aspect of this invention, a four stroke internal combustion engine comprises a plurality of generally horizontally extending, vertically spaced cylinder bores. A plurality of pistons are provided and each piston reciprocates within a respective one of the cylinder bores. A cylinder head closes one end of the cylinder bores. Each of the cylinder bores, the pistons and the cylinder head generally defines a respective combustion chamber for burning an intake charge. A crankcase member closes the other ends of the cylinders and defining at least in part a crankcase chamber in which a crankshaft driven by the piston rotates. The cylinder head has a plurality of air intake passages each communicating with a respective one of the combustion chambers for supplying at least an air charge thereto. A plenum chamber has an atmospheric air inlet juxtaposed to the crankcase member. A plurality of generally horizontally extending, vertically spaced duct members extends from the plenum chamber along one side of the engine toward the cylinder head intake passages. Throttle body means is interposed between the duct members and the cylinder head intake passages for controlling the flow of air to the combustion chambers.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.